Compensation for variations in a capacitive sense matrix

ABSTRACT

A readout device for a capacitive sense matrix includes a computer readable storage medium configured to store capacitance data. The capacitance data represents capacitance values of the capacitive sense matrix. The readout device also includes a readout circuit configured to receive a signal from the capacitive sense matrix, the readout circuit being configured based upon the capacitance data. Also described are a readout method and a method of compensating for variations in capacitance.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application from United StatesApplication for patent Ser. No. 13/629,877 filed Sep. 28, 2012, thedisclosure of which is incorporated by reference.

TECHNICAL FIELD

The techniques described herein relate generally to capacitive detectionand more specifically to capacitive detection using a capacitive sensematrix. The techniques described herein may be used in connection withtouch or proximity detection using a touch panel, for example.

BACKGROUND

A touch panel is a device that can detect an object, such as a finger orstylus, in contact with the touch panel. Touch panels can measurevarious parameters of the user's touch, such as the location at whichthe touch occurs on the panel, the duration of a touch event, etc. Touchpanels can be included in devices such as touch screens and touchpads.

A touch screen is a device that can display images and also detect anobject in contact with the screen. Touch screens are used as a userinterface in various applications such as mobile phones, tabletcomputers, etc.

A touchpad is a user interface that enables a user to provide a touchinput. Touchpads are used in applications such as laptop computers, inwhich user input is provided through the touchpad (e.g., to move acursor) and images are displayed on a separate screen.

One type of touch panel is a capacitive touch panel. Capacitive touchpanels include a capacitive sense matrix including conductive rows andcolumns. In operation, the capacitance between each row and column ofthe matrix may be detected. A change in capacitance between a row andcolumn can indicate that an object is touching the touch panel near theregion of intersection of the row and column.

SUMMARY

Some embodiments relate to a readout device for a capacitive sensematrix. The readout device includes a computer readable storage mediumconfigured to store capacitance data. The capacitance data representscapacitance values of the capacitive sense matrix. The readout devicealso includes a readout circuit configured to receive a signal from thecapacitive sense matrix. The readout circuit is configured based uponthe capacitance data.

Some embodiments relate to a readout method for a capacitive sensematrix. The readout method includes storing capacitance datarepresenting capacitance values of the capacitive sense matrix,configuring a readout circuit for the capacitive sense matrix based uponthe capacitance data, and receiving, by the readout circuit, a signalfrom the capacitive sense matrix.

Some embodiments relate to a readout device for a capacitive sensematrix. The readout device includes means for storing capacitance data.The capacitance data represents capacitance values of the capacitivesense matrix. The readout device also includes a readout circuitconfigured to receive a signal from the capacitive sense matrix. Thereadout circuit is configured based upon the capacitance data.

Some embodiments relate to a readout device for a capacitive sensematrix. The readout device includes a computer readable storage mediumconfigured to store capacitance data. The capacitance data representscapacitance values of the capacitive sense matrix. The readout devicealso includes readout means for receiving a signal from the capacitivesense matrix. The readout means is configured based upon the capacitancedata.

In an embodiment, a readout device for a capacitive sense matrixincluding a capacitive intersection associated with a force node and asense node, wherein said force node is configured to receive a drivesignal, comprises: a differential amplifier having an input and anoutput, wherein said input is coupled to the sense node; a firstcapacitor coupled between the input and output of the differentialamplifier; and a series connection of an inverter and a second capacitorcoupled between said force node and said input of the differentialamplifier; wherein said second capacitor comprises a variable capacitorhaving a variable capacitance value.

In an embodiment, a readout device for a capacitive sense including acapacitive intersection associated with a force node and a sense node,wherein said force node is configured to receive a drive signal,comprises: a capacitor, different from the capacitive intersection,coupled to the force node and having a variable capacitance set by valueobtained from capacitance data representing an expected capacitance ofthe capacitive intersection; and an integration circuit having an inputconfigured to receive a first signal from the capacitive intersectionand configured to receive a second signal from the capacitor having thevariable capacitance value, the integration circuit operating todetermine a difference between the first and second signals and generatean output signal as a function of both a sensed capacitance at thecapacitive intersection as indicated by the first signal and theexpected capacitance as indicated by the second signal.

In an embodiment, a readout device for a capacitive sense matrixincluding a plurality of first capacitors each comprised of a capacitiveintersection associated with a force node and a sense node, wherein saidforce node is configured to receive a drive signal, comprises: a memoryconfigured to store capacitance data representing an expectedcapacitance value of one or more of the first capacitors of thecapacitive sense matrix; a differential amplifier having an input and anoutput, wherein said input is coupled to the sense node; a firstcapacitor coupled between the input and output of the differentialamplifier; a series connection of an inverter and a second capacitorbetween said force node and said input of the differential amplifier;wherein said second capacitor is different from any of the firstcapacitors of the capacitive sense matrix; and wherein said secondcapacitor comprises a variable capacitor having a variable capacitanceset by the capacitance data representing the expected capacitance valueof one or more of the first capacitors of the capacitive sense matrixstored in the memory.

The foregoing summary is provided by way of example and is not intendedto be limiting. All combinations of the foregoing concepts andadditional concepts discussed in greater detail below (provided suchconcepts are not mutually inconsistent) are contemplated as being partof the inventive subject matter disclosed herein. In particular, allcombinations of claimed subject matter appearing at the end of thisdisclosure are contemplated as being part of the inventive subjectmatter disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, each identical or nearly identical component that isillustrated in various figures is represented by a like referencecharacter. For purposes of clarity, not every component may be labeledin every drawing.

FIG. 1A shows an example of a capacitive touch panel.

FIG. 1B shows a capacitive sense matrix of a capacitive touch panel.

FIG. 1C shows the capacitance between a row and a column of thecapacitive sense matrix.

FIG. 2 shows a block diagram of a detection system that includes acapacitive sense matrix and a readout device.

FIG. 3 shows an example of a readout circuit for a capacitive sensematrix.

FIG. 4 shows another example of a readout circuit for a capacitive sensematrix, according to some embodiments.

FIG. 5 illustrates a technique for storing capacitance data in memory,according to some embodiments.

FIG. 6 illustrates segments of a touch panel for which capacitance datamay be stored, according to some embodiments.

FIG. 7 is a block diagram of an exemplary computing device.

DETAILED DESCRIPTION

Described herein are readout devices and techniques for capacitive touchdetection that may be used with any of a variety of devices that includea capacitive sense matrix. The inventors have recognized and appreciatedthat the capacitance between rows and columns of a capacitive sensematrix may vary across the capacitive sense matrix due to manufacturingvariations, for example. According to some embodiments, touch detectioncan be improved by compensating for variations in capacitance across thetouch panel. For example, capacitance data representing capacitancevalues across the capacitive sense matrix can be stored, and a readoutcircuit coupled to the capacitive sense matrix can be configured basedupon the stored capacitance data.

FIG. 1A shows an example of a capacitive touch panel having conductiverows 2 and columns 3 of a capacitive sense matrix 1 arranged in adiamond pattern. When the capacitive touch panel is included in a touchscreen, capacitive sense matrix 1 may be transparent to allow light froman underlying display unit to pass through the capacitive sense matrix 1for viewing by a user. FIG. 1A also shows that a plurality of conductors4 may be provided for making contact to the conductive rows 2 andcolumns 3. In some embodiments, the conductive rows 2 and columns 3 maycover substantially the entire face of the capacitive touch panel,enabling touch and proximity detection at substantially any location onthe capacitive touch panel.

FIG. 1B shows the capacitive sense matrix 1 in further detail. Thecapacitive sense matrix 1 includes a plurality of conductive columns 3(C) and a plurality of conductive rows 2 (R). The conductive rows 2 andcolumns 3 may be formed of any suitable conductive material. Theconductive columns 3 extend vertically and the conductive rows 2 extendhorizontally as illustrated in FIG. 1B. The conductive rows 2 andcolumns 3 cross above/below each other at their region of intersection(as viewed from above) such that they are not in contact with oneanother. The conductive rows 2 and columns 3 may be separated from oneanother by an insulating material. As a result, individual row andcolumn conductors 2, 3 are separated from each other by capacitive gaps5. In this example, each of the conductive rows 2 and columns 3 hasconductors arranged in a diamond pattern. The diamond-shaped pattern ofconductive rows 2 and columns 3 may provide for increased capacitancebetween the conductive rows 2 and columns 3. However, conductive rows 2and columns 3 may be formed in any suitable shape. In operation,capacitive sense matrix 1 may detect an object that modifies thefringing electric field in the vicinity of the capacitive gaps 5 when incontact with the touch panel or in the proximity of the touch panel.

FIG. 1C shows that when a conductive column C_(i) and row R_(j) areselected, the total capacitance between column C_(i) and row R_(j) isthe sum of four capacitances 6 arising between the four adjacentdiamond-shaped regions of column C_(i) and row R_(j). The capacitancebetween column C_(i) and row R_(j) can be measured to determine whetheran object is in contact with or in the proximity of the capacitive sensematrix in which the four capacitances 6 are formed.

FIG. 2 is a block diagram of a capacitive detection system 20 thatincludes the capacitive sense matrix 1 and an associated readout device21, according to some embodiments. The readout device 21 includes acolumn switch circuit 22 for selection of one or more columns and a rowswitch circuit 23 for selection of one or more rows of the capacitivesense matrix.

During a measurement interval for a selected at least one row and atleast one column, charge sensed from the capacitive sense matrix 1 maybe integrated into a voltage and converted into a digital value bycapacitance to digital converter 25. The amount of charge stored dependson the capacitance between the row/column pair. The capacitance betweenthe selected row and column may change when an object is touching thetouch panel near the intersection area of the row and column and altersthe electric field in this region. To determine whether an object ispresent, the stored charge can be read out and converted into voltagethat is then digitized. The capacitance to digital converter 25 mayinclude an integrator 26 for converting a detected charge into a voltageand an analog-to-digital converter 27 for converting the voltage into adigital value.

By scanning the touch panel, outputs may be determined for each positionon the touch panel, and these outputs may be processed to determine theuser inputs applied to the touch panel. For example, a measurement maybe performed for each intersection of a row and column in the touchpanel. A first measurement may be performed for the intersection ofcolumn C₁ and row R₁, and a second measurement may be performed for theintersection of column C₁ and row R₂, etc. Any suitable scanningtechnique may be used. In some cases, measurements for an entire columnmay be performed in parallel. When a plurality of measurements areperformed in parallel, a plurality of capacitance to digital converters25 may be included in the readout device 21 to perform the plurality ofmeasurements at the same time. For example, when an entire column isread out in parallel, a capacitance to digital converter 25 may bepresent for each row of the capacitive sense matrix 1. In such animplementation, a total of n capacitance to digital converters 25 may bepresent. The columns of the matrix may be read out in sequence. Forexample, column 1 may be read out, followed by column 2, etc., until theentire array has been scanned. However, the columns may be read out inany suitable order. In some implementations, the columns need not beread out in parallel. For example, a serial scanning technique may beused whereby each measurement for a selected row and column is performedsequentially. In such an implementation, a single capacitance to digitalconverter 25 may be used. The techniques described herein are notlimited as to the particular scanning technique used to perform readoutfrom the capacitive sense matrix 1.

In some embodiments, the capacitance between more than one row and/orcolumn can be sensed at a time. Sensing the capacitance between morethan one row and/or column at a time may enable increasing thesensitivity with which objects can be detected in the vicinity of thecapacitive sense matrix. Sensing the capacitance between more than onerow and/or column at a time may enable detecting an object, such as afinger or stylus, that is in the proximity of the capacitances betweenthe selected rows/columns even though the object may not be in contactwith the touch panel. Thus, an approaching object may be detected whenit comes within a small enough distance of the touch panel prior to theobject coming into contact with the touch panel.

FIG. 3 shows an example of a readout circuit 30 for a capacitive sensematrix. The “force” node illustrated in FIG. 3 represents a selected rowof the touch panel to which a driving signal is applied. The “sense”node represents a selected column of the touch panel from which a signalis sensed. As discussed above, a capacitance Cm (termed a “mutualcapacitance”) is present between the selected force and sense nodes ofthe selected row and column. The techniques described herein are notlimited to the choice of a row or column as the force or sense node, asin some embodiments a column may be used as a force node and a row maybe used as the sense node.

The readout circuit 30 includes an integrator having an amplifier A anda capacitive element Cc. Amplifier A may be an operational amplifierhaving an inverting input connected to the sense node and anon-inverting input connected to a common mode voltage, e.g., ground.The inverting input may be considered to be at a virtual ground.Capacitive element Cc (e.g., a capacitor) is connected between theinverting input of the amplifier A and the output of the amplifier A. Asillustrated in FIG. 3, parasitic capacitances Cp may be present betweenthe source node and the common mode voltage and/or between the forcenode and the common node voltage. Amplifier A produces an output voltageVout that may be provided to one or more additional circuits for furtherprocessing, such as a demodulator and/or an analog to digital converter,for example.

In operation, a step voltage Vforce can be applied to the force node asa driving signal. In response to the driving signal, the output ofamplifier A produces a voltage Vout according to the followingexpression:Vout=−Vforce*Cm/Cc

The output Vout thus depends on the mutual capacitance Cm between theforce and sense nodes. When an object such as a finger contacts thetouch panel at or near the region of intersection of the selected rowand column of the touch panel, a portion of the charge stored by themutual capacitance Cm can be reduced. For example, the mutualcapacitance Cm may appear to be reduced by 10%. As a result, the voltageVout will be reduced in proportion to fraction of charge removed fromthe mutual capacitance Cm. As an example, when the value Cm is reducedby 10%, Vout is reduced by 10%. The reduction in Vout can be sensed todetect that a touch has occurred in the proximity of the region ofintersection of the selected row and column of the touch panel.

In such an implementation, the capacitance of capacitive element Cc mayneed to be selected such that Vout will not exceed the maximum outputvoltage of the amplifier A. The dynamic range is limited, as the outputvoltage swing of the amplifier A will be between about 90% and 100% ofthe maximum output voltage, in this example.

FIG. 4 shows an embodiment of a readout circuit 40 having improveddynamic range. Readout circuit 40 includes an inverter I and capacitiveelement Cx in addition to amplifier A and capacitor Cc. The input of theinverter I is connected to receive the driving signal, and the output ofthe inverter I is connected to a first terminal of the capacitiveelement Cx. The second terminal of the capacitive element Cx isconnected to the sense node, as shown in FIG. 4. In such animplementation, the value of Cx may be set equal to the expected valueof Cm. Cx may be a variable capacitance circuit that is programmed basedon one or more Cm values measured in the touch panel. For example, Cxmay be set to be equal to an average value of the values Cm across thetouch panel, or an approximation thereto.

In operation, a driving signal, such as a step voltage Vforce, can beapplied to the force node. The output voltage Vout of the amplifier A isexpressed by the following equation:Vout=−(Vforce*Cm−Vforce*Cx)/Cc

If Cx is equal to Cm, and no touch input is received, Vout=0. When atouch input occurs, Cm appears to decrease (e.g., 10%, by way ofexample). As a result, Vout=0.1*Vforce*Cm/Cc. As a result of the use ofinverter I and capacitive element Cx as discussed above, the maximumvoltage at the output of the amplifier A can be significantly decreased(e.g., to 10% of the maximum output voltage previously needed). In someembodiments, the capacitive element Cc may be reduced in size to takeadvantage of the increased dynamic range.

If there is variation in the mutual capacitance Cm at differentpositions on the touch panel, and a single value of Cx is used for allmeasurements, the output voltage Vout can deviate from its expectedvalue. The output voltage swing of Vout will increase, which reduces thedynamic range.

In some embodiments, compensation for the variation in the mutualcapacitance across the touch panel can be performed. Capacitance dataregarding the variation in capacitance across the touch panel can beobtained and stored in one or more computer readable storage mediums,such as a memory, register, etc. Based on the stored capacitance data,the capacitance of the capacitive element Cx can be modified. Forexample, the capacitance of Cx can be modified to be equal to orapproximately equal to the value of Cm measured for a particularintersection of a row and column of the touch panel. The variation inoutput voltage of the amplifier A can thereby be reduced. Thecapacitance Cx may be set by in any suitable way, such as using acontroller, for example.

FIG. 5 shows an embodiment in which a touch panel includes a pluralityof rows numbered 1-I and a plurality of columns numbered 1-J. In someembodiments, data representing the mutual capacitance between each pairof intersecting rows and columns can be stored in a memory M. Forexample, data representing the mutual capacitance between a first rowand a first column may be stored in a first location, data representingthe mutual capacitance between the first row and a second column may bestored in a second location, etc. In one implementation, as illustratedin FIG. 5, the memory locations at which the data is stored in a memoryM may correspond to the positions of the respective intersections ofrows and columns in the touch panel. In such an embodiment, the memory Mmay have a size of at least I*J*K, where K is the number of bits used torepresent capacitance information for each location. However, thetechniques described herein are not limited in these respects, as datamay be stored in any suitable way. In some embodiments, capacitance datais not stored for every intersection between a row and a column of thetouch panel. Accordingly, the amount of capacitance data stored can bereduced.

FIG. 6 illustrates an embodiment in which a touch panel is divided intoa plurality of segments. Specifically, the touch panel illustrated inFIG. 6 is shown to include nine segments. In this example, four segmentsS1-S4 correspond to the four sides of the touch panel, four segmentsS5-S8 correspond to the four corners of the touch panel, and one segmentS9 corresponds to the central portion of the touch panel. Capacitancedata may be stored for each segment that is representative of the mutualcapacitance(s) of intersection(s) between rows and columns within thesegment. Where a segment includes a single intersection, such as acorner segment, the capacitance data stored may represent a measuredvalue for the intersection or an approximation thereto. Where a segmentincludes a plurality of intersections, such as an edge segment, thecapacitance data stored for the segment may be representative of thecapacitances of the plurality of intersections in the segment, such asan average value or a median value, for example. As another example, acapacitance value may be measured from an intersection within a segment,and the capacitance data stored for the segment may represent themeasured capacitance value for the intersection chosen. When capacitancedata is stored for segments of a touch panel, any suitable number ofsegments may be used, e.g., two or more segments. In an embodiment inwhich two segments are used, a first portion of the touch panel may beconsidered as a first segment and the remaining portion of the touchpanel may be considered as the second segment.

Since Cx may be set equal to Cm for each intersection and/or segment,the stored data representing the mutual capacitance Cm may also beconsidered to represent the desired value of Cx.

Capacitance data may be stored as digital values representing a value ofcapacitance. In some cases, capacitance data may be stored in acompressed format. In one example, capacitance data may be storedrepresenting a “coarse” value of capacitance and a “fine” adjustment.For example, if the mutual capacitances in the touch panel aredetermined to be between 4.5 pF and 5.0 pF, a “coarse” value of Cx,Cx_coarse, may be set to represent the minimum capacitance value of 4.5for the panel. For each intersection or segment, a “fine” value,Cx_fine, may be stored representing a capacitance value between 0 pF and0.5 pF. The value of Cx can then be set by summing Cx_coarse and Cx_finefor a particular intersection or segment. Such a technique may enablereducing the number of bits that need to be stored, and thus the amountof storage required. The capacitance values may be represented in anysuitable way.

The individual mutual capacitance values Cm may be measured using anysuitable technique, such as those known in the art. In some cases, thevalue of Cm may be determined using the circuit of FIG. 4. With Cx setto a known value, the value of Vout depends upon Cm as specified in theequation above, and Cm can thereby be determined. The stored capacitancedata used to set Cx for individual intersections or segments may be setbased on the measured values of Cm.

In some embodiments, techniques described herein such as setting thevalue of Cx and storing capacitance data may be carried out using one ormore computing devices. Embodiments are not limited to operating withany particular type of computing device.

FIG. 7 is a block diagram of an illustrative computing device 1000 thatmay be used to implement any of the above-described techniques.Computing device 1000 may include one or more processors 1001 and one ormore tangible, non-transitory computer-readable storage media (e.g.,memory 1003). Memory 1003 may store, in a tangible non-transitorycomputer-recordable medium, computer program instructions that, whenexecuted, implement any of the above-described functionality.Processor(s) 1001 may be coupled to memory 1003 and may execute suchcomputer program instructions to cause the functionality to be realizedand performed.

Computing device 1000 may also include a network input/output (I/O)interface 1005 via which the computing device may communicate with othercomputing devices (e.g., over a network), and may also include one ormore user I/O interfaces 1007, via which the computing device mayprovide output to and receive input from a user. The user I/O interfacesmay include devices such as a keyboard, a mouse, a microphone, a displaydevice (e.g., a monitor or touch screen), speakers, a camera, and/orvarious other types of I/O devices.

The above-described embodiments can be implemented in any of numerousways. For example, the embodiments may be implemented using hardware,software or a combination thereof. When implemented in software, thesoftware code can be executed on any suitable processor (e.g., amicroprocessor) or collection of processors, whether provided in asingle computing device or distributed among multiple computing devices.It should be appreciated that any component or collection of componentsthat perform the functions described above can be generically consideredas one or more controllers that control the above-discussed functions.The one or more controllers can be implemented in numerous ways, such aswith dedicated hardware, or with general purpose hardware (e.g., one ormore processors) that is programmed using microcode or software toperform the functions recited above.

In this respect, it should be appreciated that one implementation of theembodiments described herein comprises at least one computer-readablestorage medium (e.g., RAM, ROM, EEPROM, flash memory or other memorytechnology, CD-ROM, digital versatile disks (DVD) or other optical diskstorage, magnetic cassettes, magnetic tape, magnetic disk storage orother magnetic storage devices, or other tangible, non-transitorycomputer-readable storage medium) encoded with a computer program (i.e.,a plurality of executable instructions) that, when executed on one ormore processors, performs the above-discussed functions of one or moreembodiments. The computer-readable medium may be transportable such thatthe program stored thereon can be loaded onto any computing device toimplement aspects of the techniques discussed herein. In addition, itshould be appreciated that the reference to a computer program which,when executed, performs any of the above-discussed functions, is notlimited to an application program running on a host computer. Rather,the terms computer program and software are used herein in a genericsense to reference any type of computer code (e.g., applicationsoftware, firmware, microcode, or any other form of computerinstruction) that can be employed to program one or more processors toimplement aspects of the techniques discussed herein.

The techniques and apparatus described herein are not limited in itsapplication to the details of construction and the arrangement ofcomponents set forth in the foregoing description or illustrated in thedrawings. The techniques and apparatus described herein are capable ofother embodiments and of being practiced or of being carried out invarious ways. Also, the phraseology and terminology used herein is forthe purpose of description and should not be regarded as limiting. Theuse of “including,” “comprising,” or “having,” “containing,”“involving,” and variations thereof herein, is meant to encompass theitems listed thereafter and equivalents thereof as well as additionalitems.

Having thus described several aspects of at least one embodiment, it isto be appreciated various alterations, modifications, and improvementswill readily occur to those skilled in the art. Accordingly, theforegoing description and drawings are by way of example only.

The invention claimed is:
 1. A readout device for a capacitive sensematrix including a capacitive intersection associated with a force nodeand a sense node, wherein said force node is configured to receive adrive signal, comprising: a differential amplifier having an input andan output, wherein said input is coupled to the sense node; a firstcapacitor coupled between the input and output of the differentialamplifier; and a series connection of an inverter and a second capacitorcoupled between said force node and said input of the differentialamplifier; wherein said second capacitor comprises a variable capacitorhaving a variable capacitance value.
 2. The readout device of claim 1,further comprising a memory circuit configured to store capacitance datarepresenting an expected capacitance value of said capacitiveintersection, wherein the variable capacitance value of the secondcapacitor is set in accordance with the expected capacitance value fromthe stored capacitance data.
 3. The readout device of claim 2, whereinthe capacitive sense matrix comprises a plurality of rows of conductorsand a plurality of columns of conductors, and wherein the capacitancedata represents an expected capacitance value corresponding to a segmentof the capacitive sense matrix, the segment comprising a region ofintersection of at least one row of the plurality of rows and at leastone column of the plurality of columns.
 4. The readout device of claim3, wherein the segment is a first segment and the capacitance datafurther represents an additional expected capacitance valuecorresponding to a second segment of the capacitive sense matrix, thesecond segment comprising a second region of intersection of at leastone second row of the plurality of rows and at least one second columnof the plurality of columns.
 5. The readout device of claim 4, whereinthe second region of intersection includes regions of intersection of atleast two rows of the plurality of rows and at least two columns of theplurality of columns.
 6. The readout device of claim 4, wherein thefirst segment corresponds to a corner of the capacitive sense array. 7.The readout device of claim 4, wherein the second segment corresponds toan edge of the capacitive sense array.
 8. The readout device of claim 7,wherein the first segment corresponds to a corner of the capacitivesense array.
 9. The readout device of claim 1, further comprising acomputer readable storage medium configured to store capacitance data,and wherein the variable capacitance value of the second capacitor isvariably configured based upon the capacitance data stored in thecomputer readable storage medium.
 10. The readout device of claim 9,wherein the variable capacitance value of the second capacitor isconfigured based upon capacitance data corresponding to a capacitiveportion of the capacitive sense matrix selected for readout.
 11. Areadout device for a capacitive sense matrix including a plurality offirst capacitors each comprised of a capacitive intersection associatedwith a force node and a sense node, wherein said force node isconfigured to receive a drive signal, comprising: a memory configured tostore capacitance data representing an expected capacitance value of oneor more of the first capacitors of the capacitive sense matrix; adifferential amplifier having an input and an output, wherein said inputis coupled to the sense node; a first capacitor coupled between theinput and output of the differential amplifier; a series connection ofan inverter and a second capacitor between said force node and saidinput of the differential amplifier; wherein said second capacitor isdifferent from any of the first capacitors of the capacitive sensematrix; and wherein said second capacitor comprises a variable capacitorhaving a variable capacitance set by the capacitance data representingthe expected capacitance value of one or more of the first capacitors ofthe capacitive sense matrix stored in the memory.
 12. The readout deviceof claim 11, wherein the capacitive sense matrix comprises a pluralityof rows of conductors and a plurality of columns of conductors, andwherein the capacitance data represents an expected capacitancecorresponding to a segment of the capacitive sense matrix, the segmentcomprising a region of intersection of at least one row of the pluralityof rows and at least one column of the plurality of columns.